Postnatal changes in T-type calcium current density in rat atrial myocytes

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Ca2+ signaling driving pacemaker activity in submucosal interstitial cells of Cajal in the murine colon

Interstitial cells of Cajal (ICC) generate pacemaker activity responsible for phasic contractions in colonic segmentation and peristalsis. ICC along the submucosal border (ICC-SM) contribute to mixing and more complex patterns of colonic motility. We show the complex patterns of Ca²⁺ signaling in ICC-SM and the relationship between ICC-SM Ca²⁺ transients and activation of smooth muscle cells (SMCs) using optogenetic tools. ICC-SM displayed rhythmic firing of Ca²⁺transients ~ 15 cpm and paced adjacent SMCs. The majority of spontaneous activity occurred in regular Ca²⁺ transients clusters (CTCs) that propagated through the network. CTCs were organized and dependent upon Ca²⁺ entry through voltage-dependent Ca²⁺ conductances, L- and T-type Ca²⁺ channels. Removal of Ca²⁺ from the external solution abolished CTCs. Ca²⁺ release mechanisms reduced the duration and amplitude of Ca²⁺ transients but did not block CTCs. These data reveal how colonic pacemaker ICC-SM exhibit complex Ca^2+-firing patterns and drive smooth muscle activity and overall colonic contractions.

Warmer, faster, stronger: Ca2+ cycling in avian myocardium

Birds occupy a unique position in the evolution of cardiac design. Their hearts are capable of cardiac performance on par with, or exceeding that of mammals, and yet the structure of their cardiomyocytes resembles those of reptiles. It has been suggested that birds use intracellular Ca2+ stored within the sarcoplasmic reticulum (SR) to power contractile function, but neither SR Ca2+ content nor the cross-talk between channels underlying Ca2+-induced Ca2+ release (CICR) have been studied in adult birds. Here we used voltage clamp to investigate the Ca2+ storage and refilling capacities of the SR and the degree of trans-sarcolemmal and intracellular Ca2+ channel interplay in freshly isolated atrial and ventricular myocytes from the heart of the Japanese quail (Coturnix japonica). A trans-sarcolemmal Ca2+ current (ICa) was detectable in both quail atrial and ventricular myocytes, and was mediated only by L-type Ca2+ channels. The peak density of ICa was larger in ventricular cells than in atrial cells, and exceeded that reported for mammalian myocardium recorded under similar conditions. Steady-state SR Ca2+ content of quail myocardium was also larger than that reported for mammals, and reached 750.6±128.2 μmol l-1 in atrial cells and 423.3±47.2 μmol l-1 in ventricular cells at 24°C. We observed SR Ca2+-dependent inactivation of ICa in ventricular myocytes, indicating cross-talk between sarcolemmal Ca2+ channels and ryanodine receptors in the SR. However, this phenomenon was not observed in atrial myocytes. Taken together, these findings help to explain the high-efficiency avian myocyte excitation-contraction coupling with regard to their reptilian-like cellular ultrastructure.

A low voltage activated Ca2+ current found in a subset of human ventricular myocytes

Low voltage activated (I_Ca-LVA) calcium currents including Cav1.3 and T-type calcium current (I_Ca-T) have not been reported in adult human left ventricular myocytes (HLVMs). We tried to examine their existence and possible correlation with etiology and patient characteristics in a big number of human LVMs isolated from explanted terminally failing (F) hearts, failing hearts with left ventricular assist device (F-LVAD) and nonfailing (NF) human hearts. LVA (I_Ca-LVA) was determined by subtracting L-type Ca²⁺ current (I_Ca-L) recorded with the holding potential of −50 mV from total Ca²⁺ current recorded with the holding potential of −90 mV or −70 mV. I_{Ca- LVA} was further tested with its sensitivity to 100 µM CdCl₂ and tetrodotoxin. Three HLVMs (3 of 137 FHLVMs) from 2 (N = 30 hearts) failing human hearts, of which one was idiopathic and the other was due to primary pulmonary hypertension, were found with I_Ca-LVA. I_Ca-LVA in one FHLVM was not sensitive to 100 µM CdCl₂ while I_Ca-LVA in another two FHLVMs was not sensitive to tetrodotoxin. It peaked at the voltage of −40~-20 mV and had a time-dependent decay faster than I_Ca-L but slower than sodium current (I_Na). I_Ca-LVA was not found in any HLVMs from NF (75 HLVMs from 17 hearts) or F-LVAD hearts (82 HLVMs from 18 hearts) but a statistically significant correlation could not be established. In conclusion, I_Ca-LVA was detected in some HLVMs of a small portion of human hearts that happened to be nonischemic failing hearts.

Chronic atrial ionic remodeling by aldosterone: potentiation of L-type Ca2+ channels and its arrhythmogenic significance

It is widely accepted that aldosterone induces atrial fibrillation (AF) by promoting structural changes, but its effects on the function of primary atrial myocytes remain unknown. We have investigated this point in adult rat atrial myocytes, chronically exposed to the hormone. This treatment produced larger amplitude of Ca²⁺ transients, longer action potential (AP) duration, and higher incidence of unsynchronized Ca²⁺ oscillations. Moreover, it also gave rise to increases in both cell membrane capacitance (C_m, 30 %) and activity of L-type Ca²⁺ channels (LTCCs, 100 %). Concerning K⁺ currents, a twofold increase was also observed, but only in a delayed rectifier component (I_Ksus). Interestingly, the maximal conductance (G_max) of Na⁺ channels was also enhanced, but it occurred in the face of a negative shift in the voltage dependence of inactivation. Thus, at physiological potentials, a decreased fraction of available channels neutralized the effect on G_Na-max. With regard to the effects on both C_m and LTCCs, they involved activation of mineralocorticoid receptors (MRs), were dose-dependent (EC₅₀ ∼20-130 nM), and developed and recovered in days. Neither gating currents nor protein levels of LTCCs were altered. Instead, the effect on LTCCs was mimicked by cAMP, reverted by a PKA inhibitor, and attenuated by a nitric oxide donor (short-term exposures). Both EGTA and the antioxidant NAC prevented the increase in C_m, without significantly interfering with the upregulation of LTCCs. Overall, these results show that chronic exposures to aldosterone result in dire functional changes at the single myocyte level, which may explain the link between aldosteronism and AF.

Ca(2+)-Currents in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Effects of Two Different Culture Conditions

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) provide a unique opportunity to study human heart physiology and pharmacology and repair injured hearts. The suitability of hiPSC-CM critically depends on how closely they share physiological properties of human adult cardiomyocytes (CM). Here we investigated whether a 3D engineered heart tissue (EHT) culture format favors maturation and addressed the L-type Ca²⁺-current (I_Ca,L) as a readout. The results were compared with hiPSC-CM cultured in conventional monolayer (ML) and to our previous data from human adult atrial and ventricular CM obtained when identical patch-clamp protocols were used. HiPSC-CM were two- to three-fold smaller than adult CM, independently of culture format [capacitance ML 45 ± 1 pF (n = 289), EHT 45 ± 1 pF (n = 460), atrial CM 87 ± 3 pF (n = 196), ventricular CM 126 ± 8 pF (n = 50)]. Only 88% of ML cells showed I_Ca, but all EHT. Basal I_Ca density was 10 ± 1 pA/pF (n = 207) for ML and 12 ± 1 pA/pF (n = 361) for EHT and was larger than in adult CM [7 ± 1 pA/pF (p < 0.05, n = 196) for atrial CM and 6 ± 1 pA/pF (p < 0.05, n = 47) for ventricular CM]. However, ML and EHT showed robust T-type Ca²⁺-currents (I_Ca,T). While (−)-Bay K 8644, that activates I_Ca,L directly, increased I_Ca,Lto the same extent in ML and EHT, β₁- and β₂-adrenoceptor effects were marginal in ML, but of same size as (−)-Bay K 8644 in EHT. The opposite was true for serotonin receptors. Sensitivity to β₁ and β₂-adrenoceptor stimulation was the same in EHT as in adult CM (−logEC₅₀: 5.9 and 6.1 for norepinephrine (NE) and epinephrine (Epi), respectively), but very low concentrations of Rp-8-Br-cAMPS were sufficient to suppress effects (−logEC₅₀: 5.3 and 5.3 respectively for NE and Epi). Taken together, hiPSC-CM express I_Ca,L at the same density as human adult CM, but, in contrast, possess robust I_Ca,T. Increased effects of catecholamines in EHT suggest more efficient maturation.

Lessons from the heart: mirroring electrophysiological characteristics during cardiac development to in vitro differentiation of stem cell derived cardiomyocytes

The ability of human pluripotent stem cells (hPSCs) to differentiate into any cell type of the three germ layers makes them a very promising cell source for multiple purposes, including regenerative medicine, drug discovery, and as a model to study disease mechanisms and progression. One of the first specialized cell types to be generated from hPSC was cardiomyocytes (CM), and differentiation protocols have evolved over the years and now allow for robust and large-scale production of hPSC-CM. Still, scientists are struggling to achieve the same, mainly ventricular, phenotype of the hPSC-CM in vitro as their adult counterpart in vivo. In vitro generated cardiomyocytes are generally described as fetal-like rather than adult. In this review, we compare the in vivo development of cardiomyocytes to the in vitro differentiation of hPSC into CM with focus on electrophysiology, structure and contractility. Furthermore, known epigenetic changes underlying the differences between adult human CM and CM differentiated from pluripotent stem cells are described. This should provide the reader with an extensive overview of the current status of human stem cell-derived cardiomyocyte phenotype and function. Additionally, the reader will gain insight into the underlying signaling pathways and mechanisms responsible for cardiomyocyte development.

Forced-exercise delays neuropathic pain in experimental diabetes: effects on voltage-activated calcium channels

Physical exercise produces a variety of psychophysical effects, including altered pain perception. Elevated levels of centrally produced endorphins or endocannabinoids are implicated as mediators of exercise-induced analgesia. The effect of exercise on the development and persistence of disease-associated acute/chronic pain remains unclear. In this study, we quantified the physiological consequence of forced-exercise on the development of diabetes-associated neuropathic pain. Euglycemic control or streptozotocin (STZ)-induced diabetic adult male rats were subdivided into sedentary or forced-exercised (2-10 weeks, treadmill) subgroups and assessed for changes in tactile responsiveness. Two weeks following STZ-treatment, sedentary rats developed a marked and sustained hypersensitivity to von Frey tactile stimulation. By comparison, STZ-treated diabetic rats undergoing forced-exercise exhibited a 4-week delay in the onset of tactile hypersensitivity that was independent of glucose control. Exercise-facilitated analgesia in diabetic rats was reversed, in a dose-dependent manner, by naloxone. Small-diameter (< 30 μm) DRG neurons harvested from STZ-treated tactile hypersensitive diabetic rats exhibited an enhanced (2.5-fold) rightward (depolarizing) shift in peak high-voltage activated (HVA) Ca(2+) current density with a concomitant appearance of a low-voltage activated (LVA) Ca(2+) current component. LVA Ca(2+) currents present in DRG neurons from hypersensitive diabetic rats exhibited a marked depolarizing shift in steady-state inactivation. Forced-exercise attenuated diabetes-associated changes in HVA Ca(2+) current density while preventing the depolarizing shift in steady-state inactivation of LVA Ca(2+) currents. Forced-exercise markedly delays the onset of diabetes-associated neuropathic pain, in part, by attenuating associated changes in HVA and LVA Ca(2+) channel function within small-diameter DRG neurons possibly by altering opioidergic tone.

Silencing of Cav1.2 gene in neonatal cardiomyocytes by lentiviral delivered shRNA

Cav1.2 (alpha1C) and Cav1.3 (alpha1D) L-type Ca channels are co-expressed in the heart. To date, there are no pharmacological or biophysical tools to separate alpha1D from alpha1C Ca currents (I(Ca-L)) in cardiomyocytes. Here, we established a physiological model to study alpha1D I(Ca-L) in native myocytes using RNA interference. Transfection of rat neonatal cardiomyocytes (RNC) with alpha1C specific siRNA resulted in low silencing efficiency (50-60%) at the mRNA and protein levels. The use of lentivirus shRNA resulted in 100% transfection efficiency and 92% silencing of the alpha1C gene by real-time PCR and Western blot. Electrophysiological experiments showed that the total I(Ca-L) was similarly reduced by 80% in lentivirus transfected cells. Both biochemical and functional data demonstrated high transfection and silencing efficiency in the cardiomyocytes using lentiviral shRNA. This novel approach allows for the assessments of the roles of alpha1C and alpha1D Ca channels in native myocytes and could be used to examine their roles in physiological and pathological settings.

T-type Ca2+ channel expression in human esophageal carcinomas: a functional role in proliferation

In the present study the role of T-type Ca(2+) channels in cancer cell proliferation was examined. Seventeen human esophageal cancer cell lines were screened for T-type channels using RT-PCR and voltage-clamp recordings. mRNAs for all three T-type channel alpha(1)-subunits (alpha(1G), alpha(1H), and alpha(1I)) were detected in all 17 cell lines: either alpha(1H) alone, alpha(1H) and alpha(1G), or all three T-type alpha(1)-subunits. Eleven cell lines were further subjected to voltage-clamp recordings: one, i.e. the TE8 cell line, was found to exhibit a typical T-type current while others exhibited a minimal or no T-type current. Cell proliferation assays were performed in the presence or absence of T-type channel blocker mibefradil in KYSE150, KYSE180 and TE1 cells expressing mRNA for T-type channel alpha(1)-subunits but lacking T-type current, and TE8 cells exhibiting T-type current. Only TE8 cell proliferation was reduced by mibefradil. Silencing the alpha(1G)-gene that encodes functional T-type Ca(2+) channels in TE8 cells with type-specific shRNA transduction also significantly decreased TE8 cell proliferation. The reduction of cell proliferation in TE8 cells was found to be associated with an up-regulation of p21(CIP1). Moreover, p53 silencing nearly abolished the up-regulation of p21(CIP1) resulting from mibefradil T-type channel blockade. Together, these findings suggest a functional role of T-type channels in certain esophageal carcinomas, and that inhibition of T-type channels reduces cell proliferation via a p53-dependent p21(CIP1) pathway.

Long-term and immediate effect of testosterone on single T-type calcium channel in neonatal rat cardiomyocytes

In the cardiovascular system, T-type calcium channels play an important role for the intracellular calcium homeostasis and spontaneous pacemaker activity and are involved in the progression of structural heart diseases. Androgens influence the cardiovascular physiology and pathophysiology. However, their effect on native T-type calcium currents (I(Ca,T)) remains unclear. To test the chronic effect of testosterone on the cardiac I(Ca,T), cultured neonatal rat ventricular cardiomyocytes were treated with testosterone (1 nM-10 microM) for 24-30 h. Current measurements were performed after testosterone washout to exclude any acute testosterone effects. Testosterone (100 nm) pretreatment significantly increased whole-cell I(Ca,T) density from 1.26 +/- 0.48 pA/pF (n = 8) to 5.06 +/- 1.75 pA/pF (n = 7; P < 0.05) and accelerated beating rate. This was attributed to both increased expression levels of the pore-forming subunits Ca(v)3.1 and Ca(v)3.2 and increased T-type single-channel activity. On single-channel level, the increase of the ensemble average current by testosterone vs. time-matched controls was due to an increased availability (58.1 +/- 4.2 vs. 21.5 +/- 4.0%, P < 0.01) and open probability (2.78 +/- 0.29 vs. 0.85 +/- 0.23%, P < 0.01). Cotreatment with the selective testosterone receptor antagonist flutamide (10 mum) prevented these chronic testosterone-induced effects. Conversely, acute application of testosterone (10 microM) decreased T-type single-channel activity in testosterone pretreated cells by reducing the open probability (0.78 +/- 0.13 vs. 2.91 +/- 0.38%, P < 0.01), availability (23.6 +/- 3.3 vs. 57.6 +/- 4.5%, P < 0.01), and peak current (-20 +/- 4 vs. -58 +/- 4 fA, P < 0.01). Flutamide (10 microM) did not abolish the testosterone-induced acute block of T-type calcium channels. Our results indicate that long-term testosterone treatment increases, whereas acute testosterone decreases neonatal rat T-type calcium currents. These effects seem to be mediated by a genomic chronic stimulation and a nongenomic acute inhibitory action.

Towards selective antagonists of T-type calcium channels: design, characterization and potential applications of NNC 55-0396

NNC 55-0396 is a structural analog of mibefradil (Ro 40-5967) that inhibits both T-type and high-voltage-activated (HVA) Ca2+ channels with a higher selectivity for T-type Ca2+ channels. The inhibitory effect of mibefradil on HVA Ca2+ channels can be attributed to a hydrolyzed metabolite of the drug: the methoxy acetate side chain of mibefradil is removed by intracellular enzymes, thus it forms (1S,2S)-2-(2-(N-[(3-benzoimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl hydroxy dihydrochloride (dm-mibefradil), which causes potent inhibition of HVA Ca2+ currents. By replacing the methoxy acetate chain of mibefradil with cyclopropanecarboxylate, a more stable analog was developed (NNC 55-0396). The acute IC50 of NNC 55-0396 to block recombinant Cav3.1 T-type channels expressed in HEK293 cells is approximately 7 muM, whereas 100 microM NNC 55-0396 has no detectable effect on high voltage-activated currents in INS-1 cells. Block of T-type Ca2+ current was partially reduced by membrane hyperpolarization and was enhanced at high stimulus frequency. Washing NNC 55-0396 out of the recording chamber did not reverse the T-type Ca2+ current activity, suggesting that the compound dissolves in or passes through the plasma membrane to exert its effect; however, intracellular perfusion of the compound did not block T-type Ca2+ currents, arguing against a cytoplasmic route of action. We conclude that NNC 55-0396, by virtue of its modified structure, does not produce the metabolite that causes inhibition of L-type Ca2+ channel channels, thus rendering it more selective to T-type Ca2+ channels.

Variation of T-type calcium channel protein expression affects cell division of cultured tumor cells

In this study we investigated the T-type calcium channel and its involvement in the cell division of U87MG cultured glioma cells and N1E-115 neuroblastoma cells. Using Western blot analysis, we found that expression of both alpha1G and alpha1H subunits of the T-type calcium channel decreased during conditions associated with a decrease in proliferation as evidenced by increased expression of cyclin D1, a marker for non-proliferating cells. Both serum starvation and application of mibefradil, a selective T-type calcium channel antagonist, resulted in a 50% decrease in the expression of alpha1G and alpha1H and a 700-900% increase in levels of cyclin D1 in U87MG and N1E-115 cells, respectively. Furthermore, overexpression of the alpha1H subunit resulted in a two-fold increase in cell proliferation compared to control cultures or cultures receiving an empty vector. In contrast, blocking expression of the alpha1G subunit using antisense oligonucleotides lead to a 70% decrease in proliferation of U87MG and N1E-115 cells compared to control cultures or cultures receiving a scrambled oligonucleotide. Our findings suggest that proliferation of U87MG glioma cells and N1E-115 is regulated by T-type calcium channel expression.

Difference in propagation of Ca2+ release in atrial and ventricular myocytes

Intracellular [Ca2+] ([Ca2+]i) was imaged in atrial and ventricular rat myocytes by means of a high-speed Nipkow confocal microscope. Atrial myocytes with an absent t-tubule system on 8-di- ANEPPS staining showed an initial rise in Ca2+ at the periphery of the cell, which propagated to the interior of the cell. Ventricular myocytes showed a uniform rise in [Ca2+]i after electrical stimulation, consistent with a prominent t-tubular network. In atrial myocytes, there was a much shorter time between the peak of the [Ca2+]i transient and the peak contraction as compared to ventricular myocytes. A regional release of Ca2+ induced by an exposure of one end of the myocyte to caffeine with a rapid solution switcher resulted in a uniform propagation of Ca2+ down the length of the cell in atrial myocytes, but we found no propagation in ventricular myocytes. A staining with rhodamine 123 indicated a much greater density of mitochondria in ventricular myocytes than in atrial myocytes. Thus the atrial myocytes display a lack of "local control" of Ca2+ release, with propagation after the Ca2+ release at the periphery induced by stimulation or at one end of the cell induced by exposure to caffeine. Ventricular myocytes showed the presence of local control, as indicated by an absence of the propagation of a local caffeine-induced Ca2+ transient. We suggest that this finding, as well as a reduced delay between the peak of the [Ca2+]i transient and the peak shortening in atrial myocytes, could be due in part to reduced Ca2+ buffering provided by mitochondria in atrial myocytes as opposed to ventricular myocytes.

Voltage-dependent and frequency-independent inhibition of recombinant Cav3.2 T-type Ca2+ channel by bepridil

Effects of bepridil on the low voltage-activated T-type Ca2+ channel (CaV3.2) current stably expressed in human embryonic kidney (HEK)-293 cells were examined using patch-clamp techniques. Bepridil potently inhibited ICa,T with a markedly voltage-dependent manner; the IC50 of bepridil was 0.4 micromol/l at the holding potential of -70 mV, which was 26 times as potent as that at -100 mV (10.6 micromol/l). Steady-state inactivation curve (8.4 +/- 1.7 mV) and conductance curve (5.9 +/- 1.9 mV) were shifted to the hyperpolarized potential by 10 micromol/l bepridil. Bepridil exerted the tonic blocking action but not the use-dependent block. Bepridil had no effect on the recovery from inactivation of T-type Ca2+ channels. Thus, high efficacy of bepridil for terminating atrial fibrillation and atrial flutter may be considered to be attributed, at least in a part, to the T-type Ca2+ channel-blocking actions.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Ablation of Cav2.3 / E?type voltage?gated calcium channel results in cardiac arrhythmia and altered autonomic control within the murine cardiovascular system

Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca(v)2.3 is expressed in mouse and human heart and that mice lacking the Ca(v)2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca(v)2.3 splice variants. The ablation of Ca(v)2.3 was found to be accompanied by a compensatory upregulation of the Ca(v)3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca(v)2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca(v)2.3(-/-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca(v)2.3(-/-) mice after autonomic block was in accordance with a complete abolishment of 2(nd) degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca(v)2.3(-/-) mice. We conclude that the expression of Ca(v)2.3 is essential for normal impulse generation and conduction in murine heart.

Disruption of growth hormone secretion alters Ca2+ current density and expression of Ca2+ channel and insulin-like growth factor genes in rat atria

The influence of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis on expression of low-voltage-activated (LVA) Ca2+ current in atrial tissue was investigated using spontaneous dwarf (SpDwf) rats, a mutant strain that lacks GH. Atrial myocytes from SpDwf rats express LVA and high-voltage-activated (HVA) Ca2+ currents and the Ca2+ channel alpha1-subunit genes CaV1.2, CaV2.3, CaV3.1, and CaV3.2. LVA current density decreases significantly beginning at, or shortly after, birth in normal animals; however, its density is maintained in SpDwf rats at 1 pA/pF for > or =12 wk after birth. The abundance of mRNAs encoding CaV2.3 and CaV3.2 declines with advancing age in normal atrial development, yet expression of CaV2.3 mRNA remains significantly elevated in older SpDwf animals. Quantitation of local transcript levels for mRNAs encoding IGF-I and IGF-I receptor (IGF-IR) also reveals significant differences in expression of these transcripts in atrial tissue of SpDwf animals compared with controls. In SpDwf rats, the abundance of IGF-IR mRNA remains elevated at many postnatal ages, whereas mRNA encoding IGF-I is maintained only in older animals. Physiological concentrations of IGF-I cause two- to threefold increases in LVA current density in primary cultures of atrial myocytes, and this effect is blocked by an antisense oligonucleotide targeting the IGF-IR. Thus disruption of GH production in SpDwf animals alters expression of atrial LVA Ca2+ channel and IGF genes as well as postnatal regulation of LVA Ca2+ current density, most likely acting through compensatory mechanisms via the local IGF-IR.

Drug targets in the voltage-gated calcium channel family: why some are and some are not

The L-type calcium channel antagonists have been, and continue to be, a very successful group of therapeutic agents targeted at cardiovascular disorders, notably angina and hypertension. The discovery that the voltage-gated calcium channels are a large and widely distributed family with important roles in both the peripheral and central nervous systems has initiated a major search for drugs active at other calcium channel types directed at disorders of the central nervous system, including pain, epilepsy, and stroke. These efforts have not been therapeutically successful thus far, and small molecule equivalents of the L-type blockers nifedipine, diltiazem, and verapamil directed at non-L-type channels have not been found. The underlying reasons for this are discussed together with suggestions for new directions, including fertility control, oxygen-sensitive channels, and calcium channel activators.

Induced automaticity in isolated rat atrial cells by incorporation of a stretch-activated conductance

Stretch of the atrium and sympathetic activity have been implicated as substrates for atrial fibrillation. We investigate how a model of stretch in combination with sympathetic stimulation can induce automaticity in atrial cells. We adapted our coupling clamp circuit so that a model ionic current that represents stretch-activated channels (SACs) was injected into an isolated rat atrial cell in real time. This current was calculated as ISAC= GSAC (Vm-ESAC), where GSAC and ESAC are the conductance and reversal potential of SACs and Vm is the cell's membrane potential. Repetitive automaticity was induced by a sufficiently large GSAC and this critical value of GSAC was decreased by exposure to isoproterenol. The critical value of GSAC decreased from 0.63+/-0.05 nS (mean+/-SE) in control to 0.40+/-0.07 nS in isoproterenol (P<0.05). Additionally, after exposure to isoproterenol, automaticity continued after GSAC was no longer applied and was accompanied by delayed after-depolarizations. In three cells, repetitive automaticity could not be induced at any value of GSAC. Exposure to 10 nM isoproterenol converted these cells to cells with repetitive automaticity in response to GSAC. We conclude that automaticity can be induced in isolated rat atrial cells by application of a model of SACs. Exposure to isoproterenol enhances this effect.

Characterization and regulation of T-type Ca2+ channels in embryonic stem cell-derived cardiomyocytes

T-type Ca2+ channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents (ICa,T) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). ICa,T was studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. ICa,T showed characteristics of Cav3.1, such as resistance to Ni2+ block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Cav3.1 mRNA abundance correlated (r2 = 0.81) with ICa,T. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that ICa,T is developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that ICa,T might play a role in the spontaneous contractions during CM development.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Quantitative analysis of the expression and distribution of calcium channel alpha 1 subunit mRNA in the atria and ventricles of the rat heart

Two distinct calcium currents are present in mammalian cardiac myocytes. Utilizing quantitative RT-PCR methods, we have analysed the expression patterns and abundance of four calcium channel alpha 1 subunit mRNAs in different regions of the rat heart and compared them to the known density of calcium currents recorded from rat atria. Our results show that Ca(V)1.2 is the most abundant of the four alpha 1 subunit transcripts in the rat heart. The Ca(V)1.2 message is more abundant in ventricle than in atria and does not vary in expression as a function of developmental age. Ca(V)2.3, Ca(V)3.1 and Ca(V)3.2 mRNAs are 10-100 times less abundant than Ca(V)1.2. Interestingly, Ca(V)2.3, Ca(V)3.1 and Ca(V)3.2 are expressed in both atria and ventricle. The abundance of atrial Ca(V)3.1 mRNA does not change significantly during development and remains high in older animals. In contrast, levels of atrial Ca(V)3.2 mRNA are high in embryonic tissue and at 3- and 4-weeks postnatal but become undetectable at 5 weeks. Expression of atrial Ca(V)2.3 mRNA is highest at 4-weeks postnatal and then declines gradually. We have previously documented that the LVA calcium current density is highest within 4-5 weeks after birth and then declines gradually reaching less than 30% of its maximal value at 12-14 weeks. The complex relationship between atrial LVA current density and the abundance of Ca(V)2.3, Ca(V)3.1 and Ca(V)3.2 mRNA suggests that their contribution to the cardiac LVA current may vary as a function of postnatal age.

Effect of tetramethyl pyrazine on L-type calcium channel in rat ventricular myocytes

To elucidate possible ionic mechanisms of antimyocardial ischemia and antiarrythmia of tetramethyl pyrazine (TP), we studied L-type Ca2+ currents (I(Ca.L)) in adult rat ventricular myocytes using the whole-cell patch-clamp technique. The results showed: (i) under physiological conditions, 0.25 mmol/L TP decreased amplitude of I(Ca.L) to 60.6% and this inhibition was increased with increasing concentration of TP. ID50 was 0.20 mmol/L. (ii) The Ca2+-antagonistic effect of TP was voltage-dependent. A marked negative shift of the steady-state inactivation curve was observed with long (10 s) conditioning prepulses, but not with short (350 ms) ones. (iii) The time course of inhibition during TP treatment was increased with an increase in drug concentration, and recovery from TP-induced inactivation of I(Ca.L) was slower than in control cases. (iv) Tonic block and use-dependent block with TP treatment, which was induced by increasing the frequency of stimulation, occurred. We suggest that TP inhibits the I(Ca.L) mainly by binding to inactivated Ca2+ channels. The high affinity of TP for the inactivated state of I(Ca.L) may play an important role in developing therapies for pathological conditions.

L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node

Although the neonatal sinus node beats at a faster rate than the adult, when a sodium current (I(Na)) present in the newborn is blocked, the spontaneous rate is slower in neonatal myocytes than in adult myocytes. This suggests a possible functional substitution of I(Na) by another current during development. We used ruptured [T-type calcium current (I(Ca,T))] and perforated [L-type calcium current (I(Ca,L))] patch clamps to study developmental changes in calcium currents in sinus node cells from adult and newborn rabbits. I(Ca,T) density did not differ with age, and no significant differences were found in the voltage dependence of activation or inactivation. I(Ca,L) density was lower in the adult than newborn (12.1 +/- 1.4 vs. 17.6 +/- 2.5 pA/pF, P = 0.049). However, activation and inactivation midpoints were shifted in opposite directions, reducing the potential contribution during late diastolic depolarization in the newborn (activation midpoints -17.3 +/- 0.8 and -22.3 +/- 1.4 mV in the newborn and adult, respectively, P = 0.001; inactivation midpoints -33.4 +/- 1.4 and -28.3 +/- 1.7 mV for the newborn and adult, respectively, P = 0.038). Recovery of I(Ca,L) from inactivation was also slower in the newborn. The results suggest that a smaller but more negatively activating and rapidly recovering I(Ca,L) in the adult sinus node may contribute to the enhanced impulse initiation at this age in the absence of I(Na).

Calcium currents and arrhythmias: insights from molecular biology

Calcium channels are critical to normal cardiac function. They are involved in the generation and conduction of the action potential and in contraction. Three surface membrane channels have been identified. The L-type Ca channel is most abundant and is responsible for Ca entry into the cell that triggers contraction. T-type Ca channels are most prevalent in the conduction system and are probably involved in automaticity. A newly described TTX-sensitive calcium current may be important in "boosting" or enhancing conduction and contraction. The main intracellular Ca channel resides in the sarcoplasmic reticulum and is responsible for the release of the Ca that activates contraction. Oscillatory behavior of this channel influences the sarcolemmal membrane, causing delayed aftercontractions and arrhythmias such as those seen in digoxin toxicity. The on-going molecular characterization of these channels will enhance our knowledge of their normal function and dysfunction in disease states, leading to the development of new therapeutic agents to treat arrhythmias and contractile dysfunction.

T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion

T-type Ca(2+) channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca(2+) channel have been identified, the nature of the isoform(s) supporting the cardiac T-type Ca(2+) current (I(Ca,T)) has not yet been determined. We describe the postnatal evolution of I(Ca,T) density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I(Ca,T) displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I(Ca,T) is poorly sensitive to Ni(2+) and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca(2+) channel subtypes suggest that neonatal I(Ca,T) is related to the alpha(1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I(Ca,T) to Ca(2+) signaling during the neonatal period.

Overexpression of T-type calcium channels in HEK-293 cells increases intracellular calcium without affecting cellular proliferation

Increased expression of low voltage-activated, T-type Ca(2+) channels has been correlated with a variety of cellular events including cell proliferation and cell cycle kinetics. The recent cloning of three genes encoding T-type alpha(1) subunits, alpha(1G), alpha(1H) and alpha(1I), now allows direct assessment of their involvement in mediating cellular proliferation. By overexpressing the human alpha(1G) and alpha(1H) subunits in human embryonic kidney (HEK-293) cells, we describe here that, although T-type channels mediate increases in intracellular Ca(2+) concentrations, there is no significant change in bromodeoxyuridine incorporation and flow cytometric analysis. These results demonstrate that expressions of T-type Ca(2+) channels are not sufficient to modulate cellular proliferation of HEK-293 cells.

Immunodetection of alpha1E voltage-gated Ca(2+) channel in chromogranin-positive muscle cells of rat heart, and in distal tubules of human kidney

The calcium channel alpha1E subunit was originally cloned from mammalian brain. A new splice variant was recently identified in rat islets of Langerhans and in human kidney by the polymerase chain reaction. The same isoform of alpha1E was detected in rat and guinea pig heart by amplifying indicative cDNA fragments and by immunostaining using peptide-specific antibodies. The apparent molecular size of cardiac alpha1E was determined by SDS-PAGE and immunoblotting (218 +/- 6 kD; n = 3). Compared to alpha1E from stably transfected HEK-293 cells, this is smaller by 28 kD. The distribution of alpha1E in cardiac muscle cells of the conducting system and in the cardiomyoblast cell line H9c2 was compared to the distribution of chromogranin, a marker of neuroendocrine cells, and to the distribution of atrial natriuretic peptide (ANP). In serial sections from atrial and ventricular regions of rat heart, co-localization of alpha1E with ANP was detected in atrium and with chromogranin A/B in Purkinje fibers of the conducting system in both rat atrium and ventricle. The kidney is another organ in which natriuretic peptide hormones are secreted. The detection of alpha1E in the distal tubules of human kidney, where urodilatin is stored and secreted, led to the conclusion that the expression of alpha1E in rat heart and human kidney is linked to regions with endocrine functions and therefore is involved in the Ca(2+)-dependent secretion of peptide hormones such as ANP and urodilatin.

T-Type and tetrodotoxin-sensitive Ca(2+) currents coexist in guinea pig ventricular myocytes and are both blocked by mibefradil

Under Na(+)-free conditions, low-voltage-activated Ca(2+) currents in cardiomyocytes from various species have been described either as Ni(2+)-sensitive T-type Ca(2+) current (I(Ca(T))) or as tetrodotoxin (TTX)-sensitive Ca(2+) current (I(Ca(TTX))). So far, coexistence of the 2 currents within the same type of myocyte has never been reported. We describe experimental conditions under which I(Ca(T)) and I(Ca(TTX)) can be separated and studied in the same cell. Rat and guinea pig ventricular myocytes were investigated with the whole-cell voltage-clamp technique in Na(+)-free solutions. Whereas rat myocytes lack I(Ca(T)) and exhibit I(Ca(TTX)) only, guinea pig myocytes possess both of these low-voltage-activated Ca(2+) currents, which are separated pharmacologically by superfusion with TTX or Ni(2+). I(Ca(T)) and I(Ca(TTX)) were of similar amplitude but significantly differed in their electrophysiological properties: I(Ca(TTX)) activated at more negative potentials than did I(Ca(T)), the potential for half-maximum steady-state inactivation was more negative, and current deactivation and recovery from inactivation were faster. I(Ca(TTX)) but not I(Ca(T)) increased after membrane rupture ("run-up"). Isolation of I(Ca(TTX)) by application of the bivalent cation Ni(2+) is critical because of possible shifts in voltage dependence. Therefore, we investigated whether the T-type Ca(2+) channel blocker mibefradil (10 micromol/L) is a suitable tool for the study of I(Ca(TTX)). However, mibefradil not only blocked I(Ca(T)) by 85+/-2% but also decreased I(Ca(TTX)) by 48+/-8%. We conclude that under Na(+)-free conditions I(Ca(T)) and I(Ca(TTX)) coexist in guinea pig ventricular myocytes and that both currents are sensitive to mibefradil. Future investigations of I(Ca(T)) will have to consider the TTX-sensitive current component to avoid possible interference.

Molecular and functional properties of the human alpha(1G) subunit that forms T-type calcium channels

We describe here several novel properties of the human alpha(1G) subunit that forms T-type calcium channels. The partial intron/exon structure of the corresponding gene CACNA1G was defined and several alpha(1G) isoforms were identified, especially two isoforms that exhibit a distinct III-IV loop: alpha(1G-a) and alpha(1G-b). Northern blot and dot blot analyses indicated that alpha(1G) mRNA is predominantly expressed in the brain, especially in thalamus, cerebellum, and substantia nigra. Additional experiments have also provided evidence that alpha(1G) mRNA is expressed at a higher level during fetal life in nonneuronal tissues (i.e. kidney, heart, and lung). Functional expression in HEK 293 cells of a full-length cDNA encoding the shortest alpha(1G) isoform identified to date, alpha(1G-b), resulted in transient, low threshold activated Ca(2+) currents with the expected permeability ratio (I(Sr) > I(Ca) >/= I(Ba)) and channel conductance ( approximately 7 pS). These properties, together with slowly deactivating tail currents, are typical of those of native T-type Ca(2+) channels. This alpha(1G)-related current was inhibited by mibefradil (IC(50) = 2 microM) and weakly blocked by Ni(2+) ions (IC(50) = 148 microM) and amiloride (IC(50) > 1 mM). We showed that steady state activation and inactivation properties of this current can generate a "window current" in the range of -65 to -55 mV. Using neuronal action potential waveforms, we show that alpha(1G) channels produce a massive and sustained Ca(2+) influx due to their slow deactivation properties. These latter properties would account for the specificity of Ca(2+) influx via T-type channels that occurs in the range of physiological resting membrane potentials, differing considerably from the behavior of other Ca(2+) channels.

Expression of T-type Ca(2+) channels in ventricular cells from hypertrophied rat hearts

In this study we examined the existence of T-type Ca(2+) current in ventricular myocytes isolated from rats with pressure-overload hypertrophy. The whole-cell clamp technique was used to record Ca(2+) currents in enzymatically dissociated ventricular cells. T- and L-type Ca(2+) currents were separated by applying voltage steps to different test potentials from a holding potential of -80 mV and -50 mV. T-type Ca(2+) current was defined as the difference between the currents from the two holding potentials. Ventricular myocytes from sham-operated rats showed only L-type Ca(2+) current (maximal density -13.9+/-1.3 pA/pF n=17), whereas ventricular myocytes isolated from rats with aortic stenosis showed both L- and T-type Ca(2+) currents. The average values of T- and L-type Ca(2+) current density were -4.8+/-0.4 pA/pF and -12.4+/-0.9 pA/pF (n=32), respectively. T-type Ca(2+) current was distinguished from L-type Ca(2+) current by its voltage dependence, its kinetics and by its strong blockade by nickel 50 microM. In conclusion, we have demonstrated that hypertrophied ventricular rat cells express T-type Ca(2+) channels and this finding strongly supports a role for this channel in regulating growth processes in cardiac tissue.

Low-voltage activated calcium channels: achievements and problems

Low-voltage activated Ca2+ channels, which possess unique properties quite different from those of common (high-voltage activated) channels, were discovered 15 years ago but the first alpha1 subunit has only recently been identified which might provide their structural basis. However, simultaneously, extensive data are being accumulated on the functional diversity of low-voltage activated Ca2+ currents with regard to their pharmacological sensitivity, ionic selectivity, activation and inactivation kinetics. Such diversity corresponds to equally prominent heterogeneity in the location and function of the channels. This commentary summarizes the data available in an attempt to predict a possibly wider structural subdivision of low-voltage activated Ca2+ channels into subtypes.

Angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists in the treatment of heart failure caused by left ventricular systolic dysfunction

Activation of the renin-angiotensin-aldosterone system (RAAS) in left ventricular systolic dysfunction is a critically important determinant in the pathophysiologic processes that lead to progression of heart failure and sudden death. Angiotensin II, acting at the specific angiotensin receptor (AT1-R), activates a series of intracellular signaling sequences which are ultimately expressed within the cardiovascular system as vasoconstriction and associated vascular hypertrophy and remodeling. Angiotensin converting enzyme (ACE) inhibition leads to increases in the vasodilatory peptides bradykinin and substance P and at least an initial reduction in angiotensin II concentrations. AT1-R blocking drugs prevent access of angiotensin II to the AT1-R and thus prevent cellular activation. ACE inhibitors have clearly been demonstrated through a large number of clinical trials to increase survival in congestive heart failure, primarily by reducing the rate of progression of left ventricular dilatation and decompensation. However, this beneficial effect diminishes over time. Preliminary short-term clinical studies evaluating the efficacy of AT1-R blocking drugs in the treatment of heart failure have suggested that they elicit similar hemodynamic and neuroendocrine effects as do the ACE inhibitors. The combination ACE inhibitors and AT1-R blocking drugs offer the theoretical advantage of increasing bradykinin while blocking the actions of angiotensin II, and thus possibly show a synergistic effect. Again, preliminary studies have yielded encouraging results that are difficult to interpret because neither ACE inhibitor nor the AT1-R blocking drug doses were titrated to tolerance. Pharmacological manipulation of the RAAS has led to better understanding of its role in heart failure and improved clinical outcomes.

Potent vasodilatory with minor cardiodepressant actions of mibefradil in human cardiac tissue

1. The present study compared the cardiovascular effects of mibefradil (MIB), a novel Ca2+-channel antagonist with high selectivity for T-type Ca2+-channels to the effect of the L-type Ca2+-channel-antagonists nifedipine (NIF) and diltiazem (DIL) in left ventricular myocardium and coronary arteries of hearts obtained from patients suffering from dilated cardiomyopathy (NYHA IV). Right atrial myocardium from patients undergoing aortocoronary bypass surgery without signs of cardiac failure was studied as well. 2. NIF and DIL (100 micromol l(-1)) completely depressed force of contraction (FOC) in electrically driven left ventricular myocardium (NIF 6.5+/-1.4% and DIL 7.1+/-1.2% of control), whereas a similar concentration of MIB only reduced force of contraction to 55.1+/-4.0% of the basal FOC. The negative inotropic potency as measured by the concentration needed to reduce basal FOC for 25% was NIF (0.0095 micromol l(-1))>DIL (0.041 micromol l(-1))>MIB (9.47 micromol l(-1)). 3. All three Ca2+-channel antagonists were more potent in human atrial compared to human left ventricular myocardium to reduce FOC. 4. The rank order of Ca+-antagonistic moiety as measured by the decrease of the intracellular Ca2+-transient (fura-2 ratio method) was NIF>DIL>MIB. 5. All Ca2+-channel antagonists completely relaxed human coronary arteries (% of papaverine effect: MIB 81.7+/-5.5%, DIL 91.3+/-0.9%, NIF 96.4+/-3.7%) precontracted with PGF2alpha (0.3 micromol l(-1)). The rank order of vasodilatory potency was NIF (EC50; 0.02 micromol l(-1))>DIL (0.13 micromol l(-1))>MIB (2.05 micromol l(-1)). 6. The vasoselectivity measured by the ratio of the concentration needed to achieve a 25% decrease in force and the concentration needed for 25% vasodilatation was 316 for MIB, 1.5 for NIF and 1.0 for DIL. 7. The present study provides evidence that blockade of T-type Ca2+-channels (e.g. mibefradil) results in potent vasodilatory properties with only minor cardiodepressant effects.

T- and L-type Ca2+-channel antagonists reduce contractility in guinea pig cardiac myocytes

The aim of this study was to investigate the influence of L- and T-type Ca2+-channel blockade on myocardial contractility in guinea pig cardiomyocytes. Left ventricular myocardium from guinea pig contains both L- and T-type Ca2+ channels. The T-type Ca2+ influx was inhibited with mibefradil (1-100 microM), a novel compound with a threefold higher affinity for T- compared with L-type Ca2+ channels. In comparison, L-type Ca2+ influx was reduced by the benzodiazepine diltiazem (1-100 microM). The effect of mibefradil and diltiazem on electrically driven (0.5 Hz) isolated cardiomyocytes (n = 12) was studied in a concentration-dependent manner. The change of the contraction amplitude (percentage of cell shortening) was continuously recorded with an one-dimensional high-speed camera. Both mibefradil and diltiazem concentration-dependently reduced (p < 0.05 vs. control) the contraction amplitude in isolated myocytes from guinea pig. The concentration at which the contraction amplitude of guinea pig cardiomyocytes was reduced by 50% (EC50) was 31.6 microM for diltiazem and 6.3 microM for mibefradil, indicating that the T-type Ca2+-channel blocker mibefradil is more potent in reducing contractility in guinea pig cardiac myocytes in comparison with the L-type Ca2+-channel antagonist diltiazem. Mean values for cell shortening in percentage +/- SEM for mibefradil (0, 1, 10, 100 microM) were 100%, 78 +/- 9.2%, 36 +/- 5.4%, and 24 +/- 3.6%. The corresponding values for diltiazem were 100%, 92 +/- 12.5%, 79 +/- 8.9%, and 35 +/- 2.6%. In contrast, the increase of the extracellular Ca2+ concentration (2-7.5 mM) resulted in a significant increase of the contraction amplitude (+213 +/- 14%). Therefore, blockade of the Ca2+ influx through voltage-dependent T- or L-type Ca2+ channels decreases contraction in isolated cardiac myocytes from guinea pigs containing L- and T-type Ca2+ channels.

Both T- and L-type Ca2+ channels can contribute to excitation-contraction coupling in cardiac Purkinje cells

Although L-type Ca2+ channels have been shown to play a central role in cardiac excitation-contraction (E-C) coupling, little is known about the role of T-type Ca2+ channels in this process. We used the amphotericin B perforated patch method to study the possible role of T-type Ca2+ current in E-C coupling in isolated canine Purkinje myocytes where both Ca2+ currents are large. T-type Ca2+ current was separated from L-type Ca2+ current using protocols employing the different voltage dependencies of the channel types and their different sensitivities to pharmacological blockade. We showed that Ca2+ admitted through either T- or L-type Ca2+ channels is capable of initiating contraction and that the contractions depended on Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR). The contractions, however, had different properties. Those initiated by Ca2+ entry through T-type Ca2+ channels had a longer delay to the onset of shortening, slower rates of shortening and relaxation, lower peak shortening, and longer time to peak shortening. These differences were present even when L-type Ca2+ current amplitude, or charge entry, was less than that of T-type Ca2+ current, suggesting that Ca2+ entry through the T-type Ca2+ channel is a less effective signal transduction mechanism to the SR than is Ca2+ entry through the L-type Ca2+ channel. We conclude that under our experimental conditions in cardiac Purkinje cells Ca2+ entry through the T-type Ca2+ channel can activate cell contraction. However, Ca2+ entry through the L-type Ca2+ channel is a more effective signal transduction mechanism. Our findings support the concept that different structural relationships exist between these channel types and the SR Ca2+ release mechanism.

Ion channels and early development of neural cells

In this review we underscore the merits of using voltage-dependent ion channels as markers for neuronal differentiation from the early stages of uncommitted embryonic blastomeres. Furthermore, a fairly large part of the review is devoted to the descriptions of the establishment of a simple model system for neural induction derived from the cleavage-arrested eight-cell ascidian embryo by pairing a single ectodermal with a single vegetal blastomere as a competent and an inducer cell, respectively. The descriptions are focused particularly on the early developmental processes of various ion channels in neuronal and other excitable membranes observed in this extraordinarily simple system, and we compare these results with those in other significant and definable systems for neural differentiation. It is stressed that this simple system, for which most of the electronic and optical methods and various injection experiments are applicable, may be useful for future molecular physiological studies on the intracellular process of differentiation of the early embryonic cells. We have also highlighted the importance of suppressive mechanisms for cellular differentiation from the experimental results, such as epidermal commitment of the cleavage-arrested one-cell Halocynthia embryos or suppression of epidermal-specific transcription of inward rectifier channels by neural induction signals. It was suggested that reciprocal suppressive mechanisms at the transcriptional level may be one of the key processes for cellular differentiation, by which exclusivity of cell types is maintained.

Antisense oligonucleotides against rat brain alpha1E DNA and its atrial homologue decrease T-type calcium current in atrial myocytes

Low voltage-activated, or T-type, calcium currents are important regulators of neuronal and muscle excitability, secretion, and possibly cell growth and differentiation. The gene (or genes) coding for the pore-forming subunit of low voltage-activated channel proteins has not been unequivocally identified. We have used reverse transcription-PCR to identify partial clones from rat atrial myocytes that share high homology with a member of the E class of calcium channel genes. Antisense oligonucleotides targeting one of these partial clones (raE1) specifically block the increase in T-current density that normally results when atrial myocytes are treated with insulin-like growth factor 1 (IGF-1). Antisense oligonucleotides targeting portions of the neuronal rat alpha1E sequence, which are not part of the clones detected in atrial tissue, also block the IGF-1-induced increase in T-current, suggesting that the high homology to alpha1E seen in the partial clone may be present in the complete atrial sequence. The basal T-current expressed in these cells is also blocked by antisense oligonucleotides, which is consistent with the notion that IGF-1 up-regulates the same gene that encodes the basal current. These results support the hypothesis that a member of the E class of calcium channel genes encodes a low voltage-activated calcium channel in atrial myocytes.

Molecular biology of calcium channels in the cardiovascular system

Calcium ions are key intracellular messengers in the cardiovascular system. Calcium homeostasis is regulated by an extracellular cycle, which controls the entry and removal of calcium between the cytosol and extracellular space, and an intracellular cycle, which controls calcium fluxes between the cytosol and intracellular stores in the sarcoplasmic reticulum. Several protein families mediate these calcium fluxes including those that (1) regulate the entry of calcium into the cytosol; (2) recognize calcium within the cytosol; and (3) remove calcium from the cytosol. Intracellular calcium binding proteins (the "E-F hand" proteins) recognize the appearance of calcium in the cytosol; in the heart and vascular smooth muscle, these proteins initiate excitation-contraction coupling. Calcium efflux occurs via adenosine triphosphate (ATP)-dependent calcium pumps and sodium-calcium exchangers, while two families of channels--intracellular release calcium channels and plasma membrane calcium channels--regulate calcium entry into the cytosol. The plasma membrane calcium channels, which include the L- and T-type channels, are of the greatest clinical interest because they are targets for pharmacologic therapy. T-type calcium channels, which activate contraction in vascular smooth muscle but have little or no role in cardiac excitation-contraction coupling, appear to be involved in signal transduction pathways that promote cell growth and proliferation. Calcium channel blockers that selectively block T-type calcium channels, therefore, offer a novel approach to cardiovascular drug therapy.

Protein families that mediate Ca2+ signaling in the cardiovascular system

Five protein families are known to participate in the signaling cascades that enable calcium ions (Ca2+) to regulate functions in the cardiovascular system. Ca2+ signaling is involved in muscle contraction, pacemaking, and perhaps cell growth and differentiation. Recent evidence about the molecular properties of Ca2+ regulatory proteins has suggested possibilities for new therapeutic agents, including T-type Ca2+ channel blockers for patients with cardiovascular disease. This article reviews new information about Ca2+ signaling in the heart, vascular smooth muscle, and other tissues.

Calcium channel diversity in the cardiovascular system

The flux of calcium ions (Ca2+) into the cytosol, where they serve as intracellular messengers, is regulated by two distinct families of Ca2+ channel proteins. These are the intracellular Ca2+ release channels, which allow Ca2+ to enter the cytosol from intracellular stores, and the plasma membrane Ca2+ channels, which control Ca2+ entry from the extracellular space. Each of these two families of channel proteins contains several subgroups. The intracellular channels include the large Ca2+ channels ("ryanodine receptors") that participate in cardiac and skeletal muscle excitation-contraction coupling, and smaller inositol trisphosphate (InsP3)-activated Ca2+ channels. The latter serve several functions, including the pharmacomechanical coupling that activates smooth muscle contraction, and possibly regulation of diastolic tone in the heart. The InsP3-activated Ca2+ channels may also participate in signal transduction systems that regulate cell growth. The family of plasma membrane Ca2+ channels includes L-type channels, which respond to membrane depolarization by generating a signal that opens the intracellular Ca2+ release channels. Calcium ion entry through L-type Ca2+ channels in the sinoatrial (SA) node contributes to pacemaker activity, whereas L-type Ca2+ channels in the atrioventricular (AV) node are essential for AV conduction. The T-type Ca2+ channels, another member of the family of plasma membrane Ca2+ channels, participate in pharmacomechanical coupling in smooth muscle. Opening of these channels in response to membrane depolarization participates in SA node pacemaker currents, but their role in the working cells of the atria and ventricle is less clear. Like the InsP3-activated intracellular Ca2+ release channels, T-type plasma membrane channels may regulate cell growth. Because most of the familiar Ca2+ channel blocking agents currently used in cardiology, such as nifedipine, verapamil and diltiazem, are selective for L-type Ca2+ channels, the recent development of drugs that selectively block T-type Ca2+ channels offers promise of new approaches to cardiovascular therapy.

Expression of voltage-operated Ca2+ channels in rat bone marrow stromal cells in vitro

The expression of voltage-operated Ca2+ currents (VOCCs) in bone marrow stromal cells cultured for 3-30 days has been studied by the use of the whole-cell patch-clamp technique. Both low-voltage-activated (LVA) and high-voltage-activated (HVA) VOCCs were recorded. LVA currents were first detectable after 6-7 days in culture and reached a peak of expression at 8 days, after which both the amplitude and frequency of expression of the current fell rapidly. The current was virtually undetectable in cells cultured for more than 15 days. The HVA current was detectable after 3 days in culture and reached a peak of both amplitude and frequency of expression after 1-2 weeks. This current was expressed consistently throughout the remaining culture period. In cultures treated with dexamethasone (10(-8) mol/L) peak expression of LVA currents still occurred at 7-8 days, but currents were enhanced approximately threefold. Expression of LVA currents was maintained to the end of the culture period. Expression of HVA currents was not significantly modified by treatment of cultures with dexamethasone. Examination of the biophysical and pharmacological (blockade by Ni2+ and diphenylhydantoin) properties of the LVA current in these cells suggests that they may have similarities with the LVA T currents of neuronal cells.

Changes in action potentials and ion currents in long-term cultured neonatal rat ventricular cells

A primary culture of neonatal ventricular myocytes isolated from day-old rats was established for investigating the changes in action potentials and ion currents over long periods. Cells at days 5 and 15 in culture were studied. These changes in vitro were compared with those in situ derived from the age-matched freshly isolated cells. During primary culture, quiescent cells demonstrated shortening of action potential durations (APD) resembling the developmental changes observed in situ. The beating cultured cells were not associated with APD shortening. Despite constant current amplitudes, the densities of Ca2+ currents (ICa) decreased in the quiescent cultures at later ages as a result of cell enlargement. ICa densities were maintained in the beating cultured and freshly isolated cells. Acceleration in the inactivation of ICa was observed during developments both in vitro and in situ. In addition, the densities of transient outward currents (Ito) tripled and doubled in the quiescent and beating cells during 15-day cultures. However, Ito in beating cultured cells made less contribution to APD in contrast to the quiescent cultured and freshly isolated myocytes. These findings demonstrate that electrophysiological properties differ between two types of long-term cultured cells. ICa densities remained constant in the beating cultures, suggesting that cell beating may be required for the maintenance of ICa density in developing cardiomyocytes.

Mibefradil prevents neointima formation after vascular injury in rats. Possible role of the blockade of the T-type voltage-operated calcium channel

Mibefradil is a novel calcium antagonist that is selective for the T-type voltage-operated calcium channel rather than the L type. Because T-type calcium channels are present on rapidly proliferating cells and mediate the increase of intracellular calcium induced by some growth factors, such as platelet-derived growth factor, we hypothesized that the blockade of T channels could prevent the excessive smooth muscle cell proliferation that occurs in conditions such as vascular injury. To test this hypothesis, we evaluated in rats the effects of mibefradil (which blocks both L- and T-type channels) on neointima formation after vascular injury, and we compared them with those of equihypotensive doses of amlodipine and verapamil (which block only L-type channels). Mibefradil (30 mg/kg) decreased the area of neointima formed 14 days after balloon injury by 54% (P < .001). In contrast, neither verapamil nor amlodipine had an effect despite a blood pressure reduction at least equal to that of mibefradil. The in vivo effect of mibefradil was indeed an inhibition of smooth muscle cell proliferation, as shown by thymidine incorporation experiments. This antiproliferative effect of mibefradil was also observed in vitro in smooth muscle cells stimulated by fetal calf serum. In this condition also, verapamil was ineffective. We conclude that in rats mibefradil has a potent antiproliferative effect on smooth muscle cells after vascular injury. This effect might be due to blockade of voltage-operated T channels.